This is a Great Planes 4-Step Prop Shaft Reamer for RC airplane propellers. It is designed for standard-size prop shafts (American measures). This is NOT for metric shafts. This will ream holes in 3/16", 1/4", 5/16", and 3/8" diameters.

New propellers typically have a hole that is too small to fit the engine. This reamer is the
best way to increase the hole size because it will not split the prop and it keeps the hole
perfectly centered.